In the terahertz wave frequency region, there are absorption peaks of many organic molecules of biological materials, medicines, electronic materials, and the like stemming from the structures and states thereof. Further, the terahertz waves easily penetrate materials such as paper, ceramic, resin, and cloth. In recent years, research and development have been conducted on imaging technology and sensing technology which make use of such characteristics of terahertz waves. For example, application thereof to a safe fluoroscopic inspection apparatus to replace an X-ray apparatus, to an in-line non-destructive inspection apparatus in a manufacturing process, and the like is expected.
As a current injection type terahertz wave light source, studies are made on a structure which uses an electromagnetic wave gain based on intersubband transition of electrons in a semiconductor quantum well structure. Non Patent Literature 1 proposes a terahertz wave band quantum cascade laser (QCL) in which double-side metal waveguides (hereinafter also referred to as DMWs) which are known as low loss waveguides are integrated as resonators. This element attains laser oscillation around 3 THz by light confinement at a high level and low loss propagation due to causing terahertz waves that are emitted by stimulated emission to be guided in a surface plasmon mode to a resonator structure in which metal is placed above and below a gain medium formed of a semiconductor thin film at a thickness of about 10 μm.